This application claims Paris Convention priority of DE 199 54 926.5 filed Nov. 16, 1999 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a method of correcting linear magnetic field inhomogeneities of a nearly homogeneous magnetic field B0 in the investigation volume of a magnetic resonance apparatus, wherein magnetic resonance is excited in a sample, disposed in the investigation volume, through a radio frequency pulse, at least one additional linear magnetic gradient field is applied, and a magnetic resonance signal is measured.
A method of this type is known e.g. for a nuclear magnetic resonance tomography apparatus disclosed in U.S. Pat. No. 5,391,990.
In the known method, a bipolar gradient pulse train acts on a sample after radio frequency excitation, and a series of echoes is measured and stored in a signal matrix. The (temporal) position of the echo maximum and its displacement between the lines of the matrix are determined and used to calculate correction currents for linear shim coils.
The known method has i.a. the disadvantages that the chemical shift between water and fatty constituents of the signal produces undesired signal modulation and the method cannot be applied in an acceptable manner to samples having short T1/T*2 times. In general, several iterations have to be carried out.
For this reason, there is a need for a rapid, uncomplicated direct shimming method which can be made insensitive to the influences of chemical shifts and can be successfully applied with samples having short relaxation times.
The object is achieved by a method of the above-mentioned type comprising the following steps:
An excitation radio frequency pulse is irradiated onto the sample;
A) a phase gradient Gix is applied in a predetermined direction x;
B) at a fixed time tdx after the radio frequency excitation pulse, a value Six of the resonance signal coming from the sample is measured, digitized and stored;
C) the steps A) and B) are repeated several times with systematically altered strength of the phase gradient Gix;
D) the values of the measured resonance signals Si are compared and a maximum value Sxmax is determined therefrom, to which a certain strength Gxmax of the phase gradient is associated;
E) from the strength Gxmax of the phase gradient determined in this fashion, a linear correction gradient magnetic field B(x) is determined for the predetermined direction x;
F) in subsequent measurements of magnetic resonance in the apparatus, the correction gradient magnetic field B(x) is applied to the investigation volume for homogenizing the magnetic field B0.
One single measuring point is recorded after a fixed predetermined time after each excitation rather than a complete signal echo. The time td is always the same as is therefore the dephasing due to inhomogeneities of the magnetic field B0. Through application of a phase gradient in the interval between t=0 and t=td, additional dephasing is produced which can be controlled in a defined manner. At the time of data recording, the two effects overlap. Through variation of the strength of the applied phase gradient, one obtains a maximum measuring signal when the dephasing influence of the field inhomogeneities in the respective direction is exactly compensated by the phase gradient. Only non-linear contributions and contributions perpendicular to the gradient direction remain.
One can minimize the influence of chemical shift in that the time td is selected such that fatty and water contributions in the B0 field are precisely in-phase at this relative point in time, in any case not of opposite phase.
The time td can be selected largely freely, in particular, it can be very short for samples with short relaxation times T1/T*2.
Since the applied optimum phase gradient corresponds directly to the correction gradient, iterations are generally not required. Parameters which must be known in other methods, such as the position of the sample in the investigation volume, the resonance frequency or the applied RF performance, must not be exactly determined.
The method has been initially described with reference to one linear dimension but can be easily extended to several dimensions by carrying it out e.g. analogously for a further predetermined direction y which is preferably perpendicular to the direction x. It is not thereby necessary, however possible and preferred, if the above described correction for the x direction has already been carried out. In general, the effects simply add. Compensation in two dimensions is particularly favorable for examination of slices of an object. In multiple slice examinations, the field can be homogenized separately for each slice.
It is of course possible to extend the method in a corresponding manner to three-dimensional volumina by carrying it out analogously for a further predetermined direction z which is preferably perpendicular to the directions x and y. Therein, the field can be homogenized in the entire sample volume or, in connection with volume-selective measures, also for selected partial volumes and possibly for many different volumes within an object.
The easiest way of carrying out the method is to use the maximum measured signal of each respective Six, Siy or Siz as the respective signal maximum Sxmax, Symax, Szmax.
Determination of a more exact, interpolated position is possible if each respective maximum of a smooth function Sx(Gx), Sy(Gy), Sz(Gz) is used as signal maximum Sxmax, Symax, Szmax which is fitted to the measured gradient strengths in the region of the maximum, preferably by a Gaussian function.
If the correction gradient field is to be determined very precisely and yet within a short time, it is recommended to initially carry out the method in rough gradient steps and then to repeat the steps A) to E) in the vicinity of the determined signal maxima, at least for one direction (x,y,z), thereby determining a refined value of the signal maxima.
A further possibility of increasing the accuracy consists in repeating, at least for one direction (x,y,z), the steps A) to E) for a different, preferably extended tdx, tdy or tdz.
Limitation to a narrow range about the already determined preliminary maximum is thereby possible. Longer td means a longer dephasing time through the remaining inhomogeneities. Since one homogenization step has been carried out already, the signal strength is still sufficient.
The method is preferably a method of nuclear magnetic resonance and, in particular, is incorporated in a method of magnetic resonance imaging. Nuclear magnetic resonance imaging apparatus and, to an increasing extent, high-resolution nuclear magnetic resonance spectroscopy apparatus have gradient coils and shim systems as standard equipment. The method can be integrated into the existing software of, in particular, imaging apparatus without any hardware problems. In particular, it can be added to the actual measurement or be completely integrated therein such that the field is newly homogenized during the investigation program. As mentioned above, homogenization can be carried out individually for separate partial investigation areas.
In a preferred embodiment of the invention, the method is carried out for two preferably orthogonal directions, e.g. x,y and is preceded by a slice selection step, optionally including step A), which selects a slice in the investigation object perpendicular to a direction e.g. z. The selected slice can, for the general case, also be inclined to designated axes.
As an alternative, the method follows a previous volume selection step, optionally including step A), which selects a volume chosen from the investigation object.
With multiple volume experiments or multiple slice experiments, several sets of correction gradient fields are determined which are associated with different volumes or slices.
Correction currents can be determined from the determined correction gradient magnetic fields for feeding into shim coils of the apparatus.
Alternatively or additionally, offset currents can be determined from the determined correction gradient magnetic fields for feeding into gradient coils of the apparatus thereby permitting alteration of said currents in a particularly easy fashion, even during a pulse program.
In an embodiment, the sample comprises biological tissue. The above-mentioned advantages of the invention have a particularly positive effect with inhomogeneous samples having fatty and water constituents whose exact position is possibly not known. The same is true for biological samples such as test animals or also human patients.
Further advantages can be extracted from the drawing and the description. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration, rather have exemplary character for describing the invention.
The invention is shown in the drawings and is explained in more detail by embodiments.